1. The Field of the Invention
The invention relates generally to the field of displacement sensors and, more specifically, the use of proximity sensors to determine the position of a displaceable element moving transversely with respect thereto.
2. The Background Art
In measurement systems, precision often stands opposed to large displacements. Measurement of large displacements or large angles, such as for motion control, typically must sacrifice precision as the range of displacement increases.
In satellite systems, because features such as space, weight, accuracy, isolation, non-contact, power, and the like are at a premium, the ability to measure comparatively large distances is limited. Conventional systems, such as linear transducers and other mechanisms to measure distance, are exceedingly bulky, heavy, and slow, and cause frictional losses and the like.
Displacement measurements are made by a plethora of mechanisms. In certain applications, particularly those in small satellites, space, weight, precision, and power are all at a premium. Oftentimes, isolation is required, and various sources of frictional loss must be minimized. Therefore, one need is for a very precise, non-contact, measurement system, over comparatively large distances. In fact, a disproportionately great incremental sensitivity of the measurement system is needed to detect small changes in displacement over comparatively large ranges of displacement.
Often, measurement is done with optical encoders to improve precision. For example, such systems in rotation require bearings and well-defined centers of rotation. The bearing sets must be extremely precise, and they add substantial energy loss due to friction. What is needed is a system that can provide both measurement of a large displacement, and comparatively fine precision, particularly for controlling motion without those resource commitments and costs.
For example, in small satellite designs, flexible pivot systems save space and energy, but cannot use optical encoders. The wear, galling, friction, and energy loss due to various mounts and bearing systems in optical encoder systems consume too many weight, energy, and data processing resources for small satellite applications.
Error arises in mechanical systems from distortion, displacement, misalignment, vibration, and other mechanical sources of error in manufacture, installation, operation, and environmental control. Moreover, mechanical devices have very practical, inherent limitations on their accuracy and precision in fabrication, assembly, and operation. What is needed is a system that is tolerant of such mechanical errors as common as misalignment, wobble, and vibration of mechanical elements. As in all mechanical systems, vibration and other oscillatory motions need to be accommodated somehow, but doing so can consume excessive and unavailable power and data processing resources, in addition to cost and complexity.
With respect to absolute distance between a surface and a proximity sensor, these errors must somehow be engineered out. However, what is needed is a mechanism to automatically compensate for such errors, in order to provide real time data that can be compensated to remove errors, and thus remove the need to process data before using it. For example, in a high speed response, a physical element may need control signals to operate at a very high frequency or band width. If data must be first processed by a computer before being used, that processing time delays the response time. For control systems, it would be advantageous if an electrical connection scheme could be devised in order to cancel errors directly as they are made.
Proximity sensors have substantial precision in detecting their relative distance to a target object. In fact, proximity sensors have sufficient precision, that they can detect comparatively very fine changes in their distance to, for example, a metal object. Their range of motion is necessarily limited to comparatively small overall displacement ranges, however, because the phenomena on which they rely decay rapidly with distance. Thus, the very sensitivity they provide renders them ineffectual at distances greater than the order of magnitude of the operational faces of such sensors.
Thus, what is needed is an apparatus and method to provide for comparatively large displacements, disproportionately precise measurement of those displacements, and immediate correction of errors due to vibration and other mechanical sources of measurement errors.